​Positive ​charge ​Q ​is ​distributed ​uniformly ​along ​the x ​axis ​from x ​= ​0 ​to x = a.​
(a) ​Determine ​the magnitude ​of ​the ​electric ​field ​at x = l + a ​on ​the x ​axis.
​(b) ​Find ​an ​expression ​for ​the ​field ​if l≫a.